In high school science class one time, we boiled up some milk and then added some acid. I think it was hydrochloric acid, which is essentially the sort of acid in your stomach, but I don't remember exactly. This caused the milk to curdle, forming little lumps of solid matter, or curds. We filtered the result to separate the curds from the runny leftover material (which is called whey). There may have been some other process in there somewhere, but at the end of it all we had extracted this solid material from the milk. We pressed it into a ball, and found that it was bouncy, like a rubber ball. It also had a faint smell of cheese.

This is because these steps are pretty much the first steps in making cheese. What we had done is to separate the protein and fat from the liquid in the milk. The main protein in cow's milk is one called casein.[1] In cheese production, the curdling is normally done not with hydrochloric acid, but with a complex of enzymes called rennet.

Rennet used to be extracted from the lining of calves' stomachs (presumably while the rest of the calf was being turned into veal). Nowadays, most of the rennet used for cheese-making is produced by genetically engineered microbes. Bacteria of a few different species have been genetically modified to produce the same enzymes that are produced by calves' stomachs. After producing a vat full of enzymes, the bacteria are filtered out, leaving just the enzymes. The resulting rennet contains none of the bacteria and is chemically identical to rennet produced from a calf. Alternatives to rennet also exist, such as similar enzymes produced by various plants, which are used by a handful of cheese makers.

Either way, you separate the milk protein and fat to give you curds. Depending on the type of cheese you are making, the next steps in the process can vary greatly. For cottage cheese, you're pretty much done - you package the moist curds and sell them, or just eat them. For other cheeses, you drain and press the curds to remove more of the moisture. At this point you could just press them into a mould, set them aside for a while, and let nature take its course. After a certain maturing period, the resulting mixture will have ripened through the action of whatever micro-organisms are around, to produce a different type of cheese.

Washed rind cheeses being ripened.

Other options with the curds are to physically process them in various ways. Chopping and kneading them with a bit of added salt is a process called cheddaring, and if you do this before pressing the mixture into a mould you end up with, as you might guess, cheddar cheese. If you knead and stretch the curds, you impart a stringy texture to the resulting cheese, giving you cheeses like mozzarella or provolone.

The longer you mature a cheese, the stronger its flavour becomes. The micro-organisms in the ripening cheese convert the raw milk proteins, sugars, and fats into complexes of simpler amino acids and fats. A mild cheese like Brie is ripened for only a few weeks. A strong cheddar can take a couple of years or more to mature.

Another thing that influences the type of cheese you produce is the type of micro-organisms which find their way into the curds. Different geographic regions have different mixes of strains and species of bacteria, moulds, and yeasts, so the cheeses they produce are all slightly different. In some cases, specific bacteria, moulds, or yeasts are added deliberately to the cheese to achieve a certain result. In this way the cheese can end up with a rind of edible mould like Brie and Camembert, or blue veins running through it, or holes like Emmentaler. The holes are actually gas bubbles, where carbon dioxide produced by the bacteria collects during ripening.

Cheeses can also vary by how they are treated during the ripening process. Some cheeses are simply left alone in a cool place. Others are washed repeatedly in water, which makes a milder cheese, while others are washed in a brine solution, which affects the saltiness and texture. Speaking of salt, more or less of it can be added to the curds to achieve a specific level of saltiness of the final cheese. Some cheeses are exposed to air, while others are sealed. Some are wrapped in leaves or rolled in ash. You don't even have to start with cow's milk - buffalo milk is traditionally used for mozzarella, goat's milk makes different types of cheese again, and I'm sure a few other types of milk are used.

The cheese making process has thousands of subtle variations, which give rise to the thousands of types of cheese available across the world. Yum!

[1] Our science teacher told us it was pronounced "say-seen", but apparently, as I have just learnt, that's not how most people say it (it's marked as "kay-seen" in both Wikipedia and my dictionary). I've had the wrong pronunciation in my head for decades (where "wrong" is defined as "not how the vast majority of people say it"). You really should be careful about these things if you're a science teacher. Actually, on reflection, that science teacher told me a lot of weird pronunciations. And they stick when you're a kid keen to learn about science and commit these things to memory. It's why I still say "cephalopod" with a hard "c": "kephalopod". (The English word "cephalopod" derives from the Greek "kephalopoda", so one could argue that it really should be a hard "k" sound at the beginning. That's the excuse I use whenever anyone tries to correct me.)